1. Field of the Invention
The present invention relates in general to the field of micro/nano-structures, and, more precisely, it relates to a method for developing fractal-branching, hierarchical structures that grow in a three-dimensional pattern.
2. Description of Prior Art
In nature, smart surfaces with highly desirable functionalities are abundant, and scientists are increasingly identifying them and delving into their exact composition in an attempt to emulate these functionalities artificially. Just a few examples include the following: a) the lotus leaf for its self-cleaning and water-repellent properties for use as coatings on buildings, windows, and solar cells; b) a gecko's toe hairs for its directional dry adhesion, in addition to its self-cleaning properties, as potential smart adhesives; and c) a shark's scales for its protection against biofouling and reduced drag to employ on ship hulls or enhanced performance swimsuits.
The examples given above are by no means an exhaustive list (e.g., consider the non-wetting surfaces of a beetle's back and a butterfly's wings, and the wet adhesion system of some insects' footpads or a frog tree's toes). While the exact morphology and material composition of the surfaces varies for all these cases, there are similarities among all of them. The most notable is the use of a hierarchy of structures from the macro- to nano-size scale. The research studies with a focus on the structural (as opposed to material) composition of the three particular examples given above, as well as the attempts to mimic these attributes artificially, are discussed in more detail below to illustrate the significant role that a structural hierarchy plays in achieving all these functionalities.
To this point, the lotus leaf, which has been studied extensively particularly in recent years, achieves its self-cleaning and water repellent properties in part from the morphology of its surface. Its surface consists of a combination of micro-sized bumps coupled with nano-sized hairs. Numerous methods have been employed to mimic this structure to achieve the same kind of superhydrophobicity including replication of an actual lotus leaf surface by molding it using poly(dimethylsiloxane) (PDMS) [Sun, et al., Langmuir, 2005]. However, further studies led to hypothesis and testing of the principles that the morphology played in achieving this superhydrophobic phenomenon. From this, it was determined that the use of micro-bumps covered with nano-hairs is actually what minimizes the surface contact with the water droplet (as opposed to solely the nano-hairs or micro-bumps) [Bhushan, et al., Phil. Trans. R. Soc. A, 2009]. Many artificial methods for generating these two-level hierarchical structures (the micro-bumps with nano-hairs) were formulated (e.g. see [Bhushan et al., US Patent No. 2010/0028604 A1, 4 Feb. 2010] and [Zhang, et al., International Patent No. WO 2009/002644 A2, 31 Dec. 2008]), and, it was also shown that the contact angle of a water droplet on the surface could also be manipulated systematically by varying the structure of the nano-sized structures on the micro-bumps [Jeong, et al., Langmuir, 2006]. Further understanding of the principal indicated that to obtain the dynamics of the water rolling easily off the surface required not only a large static contact angle but also a low contact angle hysteresis. This led to tailoring of surfaces to optimize this feature with extensions to larger temperature ranges for the generation of surfaces that resist ice formation [Mishchenko, et al., ACS Nano, 2010]. It should be noted that in this case, however, they only used one-level of micro-patterning the surface and just manipulated the shape or clumping tendencies of these one-level structures.
Similar scrutiny of the principles behind the dry adhesion properties of a gecko's toe surface and the reduced drag of a shark's skin has to lead to other surface manipulations in attempts to emulate these properties. In the case of the gecko hairs, dry adhesion is achieved due to van der Waals forces as the tips of the hairs make close contact with the surface to which they are sticking [Autumn, et al., PNAS, 2002]. This intimate contact, however, is accomplished in part due to the fractal nature of the tips of the hairs, which results in a graduated compliance from root to tip that allows good conformity. As was similarly discovered in the attempts to mimic the surface of a lotus leaf, the best performance in achieving the gecko-type adhesion came from structures with some sort of hierarchy [Lee, et al., Langmuir, 2009] but friction enhancement over a flat surface was also demonstrated with even a one-level patterned surface [Majidi, et al., Phys. Rev. Letters, 2006]. In the case of the shark's skin, the drag reduction comes from small riblet-shaped scales known as dermal denticles aligned in the fluid flow direction. An attempt to mimic this pattern for a high performance swimsuit has been also done. Again, a hierarchy between a wider weave and individual threads was used [Dean and Bhushan, Phil. Trans. R. Soc. A, 2010].
The principles of nanostructuring the surface of a material have been employed to advantage in other kinds of applications as well (aside from biomimetic ones) such as with metal oxides (e.g., nanostructuring of TiO2 potentially enhances its photocatalytic activity [Zhu, et al., J. Nanopart. Res., 2010] as is also the case with the free-standing metal oxide structures described in [Ren et al., International Patent No. WO 2004/050547 A2, 17 Jun. 2004]). Also nanowires, derived from gecko hair principals, have been used to create active-matrix circuitry for low-voltage, flexible artificial skin [Takei, et al., Nature Materials, 2010]. On the nanoscale alone, for electronic and light-related applications like solar cell arrays and light emitters/detectors, methods have been developed for growing tree-branched nanowires as shown in [Fonseca, et al., U.S. Pat. No. 7,528,060 B1, 5 May 2009] where the branched wires were formed by exposing a porous silicon wafer to a Transmission Electron Microscope (TEM) beam, in [Lee, et al., US Patent No. 2010/0084628 A1, 8 Apr. 2010] where parasitic branches are formed by a wet-etching process and/or thermal energy irradiation, and in [Samuelson, et al., U.S. Pat. No. 7,875,536 B2, 25 Jan. 2011] where nano-whiskers are induced from seeded catalytic particles via a VLS (Vapor-Liquid-Solid) method. However, it should be noted that all of the artificial attempts of biomimetic smart surfaces that were actually fabricated at most demonstrate a two-tier hierarchy (one micro-level and one nano-level) and thus lack the fractal branching structure and numerous hierarchical levels as witnessed, for instance, at the tips of real gecko hairs. While methods for growing fractal-like branched nanowires on surfaces, (e.g., as shown in [Fonseca, et al.] and [Samuelson, et al.] described above) have been proposed for other applications, they have not been successfully applied to the fabrication of these biomimetic smart surfaces. Also, they have only been shown to grow on flat surfaces (and thus only formed a layer of structures on the nano-scale).